Eldora Mountain Resort has the closest, regular nordic skiing to Boulder. Located about 30 minutes west of Boulder near the town of Nederland, Eldora typically grooms starting in November or December and goes into April.
Devils Thumb Ranch has ~80km of groomed trails nestled in the pristine and private Ranch Creek valley at the base of the Continental Divide, in Tabernash, CO. Whether you enjoy skate, classic or snowshoeing; there are rewarding experiences waiting to happen.
Crested Butte Nordic has beautiful trails and great nordic skiing and a strong community who gets out to ski together.
Steamboat Springs, a destination unto itself.
Colorado Cross Country Ski Association is an umbrella organization for most Colorado cross-country areas that is a great resource for nordic skiing.